The present invention relates to a method of making very low voltage Zener diodes--that is, Zener diodes in a range of 2.4-3.3 V at 5 mA--using epitaxial deposition.
Alloying methods have previously been used to obtain PN junctions with breakdown voltages below 3.3 V. As commonly known, alloyed junctions are not uniform and Zener diodes formed by such methods have tendencies to breakdown at the surface. To overcome surface breakdown problems, a diffused junction has to be formed prior to the formation of the alloyed junction to serve as a guard ring. The provision of a diffused junction involves an extra step of processing.
It is also known to form PN junctions for Zener diodes by diffusion. This technique is disclosed, for example, in the U.S. Pat. No. 3,723,832 to Bachmeier. However, it is not possible by diffusion to obtain an abrupt PN junction which is necessary for low breakdown voltages.
Selective epitaxial deposition has been used to create PN junctions yielding Zener voltages which are lower than the voltages obtainable by diffusion. However, conventional epitaxial deposition techniques still do not yield Zener diodes with very low Zener voltages; for example, Zener diodes in the range of 2.4-3.3 V at 5 mA.